When an implantable cardiac stimulation and/or sensing lead is placed on the outer surface of the heart in a process known as epi-myocardial lead implantation, the heart is made accessible by entering through the chest wall. The two most commonly used procedures have been the left lateral thoracotomy (exposing the left ventricle (LV)) and the subxiphoid (or subcostal) approach (allowing direct access to the apical area of the right ventricle (RV)).
The left lateral thoracotomy (or “mini thoracotomy”) technique requires an incision usually in the left fourth or fifth intercostal space. The cut-down is made sufficiently large to allow adequate access to the left side or left ventricle (LV) of the heart. The incision can be long or short (physician's preference) and is made from a point near the left parasternal border to a point near the left anterior axillary line (between the sternum and the armpit). The LV is fairly well accessible with this approach. Either screw-in sutureless myocardial leads or suture-on epicardial leads have generally been used in this approach.
The subxiphoid or subcostal approach allows access to the heart without a formal or full thoracotomy. In this approach the incision is made just below the rib cage (subcostal approach) or just below the xiphoid (subxiphoid approach). With this latter approach especially, screw-in sutureless myocardial leads can easily be placed near the apex of the RV and in some cases, on a small, limited portion of the LV.
These epi-myocardial lead implant approaches, although having potential for some significant morbidities, have been suitable over the years for the relatively few patients requiring such leads (usually pediatrics and the few adults having mechanical valves which contra-indicate use of conventional transvenous leads). However, with the rapid growth of cardiac resynchronization therapy (CRT) in congestive heart failure (CHF) patients, there is now a need to be able to reliably and quickly place a lead with a myocardial electrode on the LV. The reason for this is that in a certain percentage of CRT patients (ranging from about 5 to 15%) subjected to the current transvenous approach of implanting leads into the coronary veins (1) the coronary sinus os (the opening into the coronary sinus from the right atrium) cannot be cannulated to allow a lead to be inserted into the coronary sinus and coronary veins, or (2) the coronary venous anatomy is too tortuous for the lead to be able to be positioned into the appropriate LV coronary vein target site, or (3) the lead's electrode lands in a coronary venous site with poor, unacceptable parameters, for example, high pacing thresholds, phrenic nerve stimulation, and/or poor activation sequence/poor hemodynamics. Thus, there is a major need in the clinical realm to use myocardial leads via a transthoracic approach to place lead electrodes on the LV in such patients.
Conventional implant approaches using current epi-myocardial leads have disadvantages including the relatively large incisions needed to gain access into the thoracic cavity and to the heart's LV; the difficulty of quickly and easily attaching the lead; the higher rate of morbidity, trauma and pain to the patient; the occurrence of undetectable bleeding in the thoracic cavity due to the larger incisions; the tendency to require longer recovery time in the hospital; and cosmetic disagreeability to the patients.
Hand manipulated tools called lead introducers are used to implant screw-in or helix fixation electrodes in the cardiac tissue. Presently available lead introducers for implanting the helix electrodes of myocardial pacing leads tend to be large diameter, stiff structures designed to handle myocardial leads having lead bodies that extend radially outwardly from an electrode header. Examples of this kind of tool and lead body/header interconnection are disclosed in U.S. Pat. No. 4,972,847 issued Nov. 27, 1990, and titled “Pacing Lead and Introducer Therefor.” While lead bodies that extend radially outward from the electrode header at the junction thereof tend to conform more closely to the outer surface of the heart after implantation, the lead body is often subjected to high stresses during implantation because of the sharp bend imposed upon the lead body when the electrode header is temporarily held inside the introducer. In addition, the distal portion of the lead body extending from the electrode header is usually captured in a longitudinal groove extending along the length of the introducer thereby causing the lead to wrap around the outside of the introducer as it is rotated to advance the helix electrode into the myocardium. Introducers of this kind also tend to be relatively complex, comprising multiple parts that must be unlocked to release the lead and its electrode header from the introducer following implantation.
Myocardial leads whose lead bodies extend axially from the electrode header are also available. Although these kinds of leads tend to be more easily implanted, the lead bodies are often subjected to tight bends following implantation in order for the lead body to lie against the outer surface of the heart and follow the contour thereof. Such orientations can result in high stresses and fatigue damage at or near the lead body/electrode header interface.